1. Field of the Invention
The present invention relates to a display apparatus, employing a subfield drive system, and more particularly to the suppression of pseudo-contours in such a display apparatus.
2. Description of the Related Art
The subfield drive system is used in, for example, display apparatus having a plasma display panel (PDP). PDP displays are currently employed in large-screen, flat-panel television sets. Conventional PDP display apparatus of this type, as disclosed in Japanese Unexamined Patent Application Publication No. 10-116053, is described below.
FIG. 13 shows a conventional PDP display apparatus 100 having a pair of input terminals 1, 2 that receive an analog picture signal and a synchronizing signal, respectively. An analog-to-digital converter (ADC) 3 digitizes the picture signal. A code converter 11 converts the digital picture signal to a coded signal representing subfield patterns. A field memory 14 stores the subfield patterns for two fields. A driver 15 reads the subfield patterns from the field memory 14 and drives a PDP 16. A controller 18 controls the analog-to-digital converter 3, field memory 14, and driver 15 according to the synchronizing signal.
The input analog picture signal is, for example, a video signal comprising a series of frames, each made up of an interlaced pair of fields. The analog-to-digital converter 3 converts the analog gradation value of each picture element (pixel) to an eight-bit code in which the eight bits (b7, b6, b5, b4, b3, b2, b1, b0, in order from the most significant bit) are weighted according to powers of two (128, 64, 32, 16, 8, 4, 2, 1). This enables two hundred fifty-six gradations (0 to 255) to be expressed.
If the eight-bit digital picture signal were to be stored directly in the field memory 14, without code conversion, the driver 15 and PDP 16 would operate as illustrated in FIG. 14. FIG. 14 shows a single field display interval divided into eight subfields (SF0 to SF7). Each subfield includes an addressing interval (X) and a sustaining discharge interval (hatched). The addressing intervals all have the same length, but the lengths of the sustaining discharge intervals vary in proportion to the bit weights of bits b0 to b7. During the addressing interval of subfield interval SF0, the driver 15 reads the b0 data for all pixels in the field (the b0 bit plane of the field) from the field memory 14, and writes the b0 data into the PDP 16. The PDP 16 is of the alternating-current (AC) type and has a memory feature, retaining the b0 data for each pixel until the entire b0 bit plane has been written. During the ensuing sustaining discharge interval (not visible for subfield SF0 in FIG. 14) the pixels with ‘1’ data emit light. The other bits (b1 to b7) are processed in the same way, the length of the sustaining discharge interval doubling at each bit plane. The total amount of light emitted by each pixel in the PDP 16 is thus proportional to the luminance gradation expressed by the eight-bit data. The human visual system integrates the emitted light so that a picture with the intended gradation levels is perceived.
If the picture is a moving picture with smoothly varying gradations, however, the viewer may also perceive unintended colored bands, or bands that are lighter or darker than their neighbors. These bands, referred to as pseudo-contours, are a major factor degrading the picture quality of a moving picture displayed on a PDP. The reason for their occurrence is explained in FIGS. 15 and 16.
FIG. 15 schematically shows part of one raster line of a picture that is moving to the left on the screen. The horizontal axis indicates pixel position; the vertical axis indicates time. In one field, shown in the upper half of FIG. 15, five consecutive pixels in the raster line have gradation value 127 (represented by bit data ‘01111111’), and the next few pixels have gradation value 128 (represented by ‘10000000’). In the following field, shown in the lower part of FIG. 15, this pattern has moved two pixels to the left. As the picture moves, the viewer's eye tends to track the motion, so that light emitted from all points on dotted line R0 impinges on a single point on the viewer's retina. The same is true of lines R1 and R2.
FIG. 16 shows the relationship between retinal position and perceived luminance gradation. Point R0 is perceived with the correct gradation value of 127 and point R2 with the correct gradation value of 128, but point R1 appears to have substantially zero luminance. If the same pattern occurs in other raster lines as well, it is perceived as a vertical pseudo-contour moving to the left.
The cause of the pseudo-contour is that around point R1, the motion of the picture is accompanied by a ‘rollover’ in which bits b0 to b6 change from ‘1’ to ‘0’ and bit b7 simultaneously changes from ‘0’ to ‘1’. A similar pseudo-contour would be perceived if the picture were moving toward the right, with bits b0 to b6 changing from ‘0’ to ‘1’ and bit b7 from ‘1’ to ‘0’. Strictly speaking, a rollover is said to occur whenever an increment or decrement of one gradation level produces a carry or borrow at any bit position, so that one bit changes from ‘0’ to ‘1’ and another bit changes from ‘1’ to ‘0’. Pseudo-contours are most noticeable when there is a rollover involving the most significant bit (b7).
If analyzed further, the pseudo-contour phenomenon is found to occur when a gradation change is accompanied by a large shift in the temporal center of gravity of light emission and a large shift in the sustaining discharge intervals during which light is emitted. In FIG. 15, for example, for gradation 127, light emission is concentrated in the first half of the field interval, while for gradation 128, light emission is concentrated in the second half; when the gradation value changes from 127 to 128, all sustaining discharge intervals in the first half of the field interval change from emitting light to not emitting light, and all sustaining discharge intervals in the second half of the field interval (the single sustaining discharge interval of subfield SF7) change from not emitting light to emitting light.
To mitigate the deterioration of moving-picture quality due to pseudo-contours, the code converter 11 in FIG. 13 converts the eight-bit digital code (b0 to b7) output from the analog-to-digital converter 3 to a nine-bit digital code (bb0, bb1, bb2, bb3, bb4, bb5, bb6, bb7, bb8), and the driver 15 divides the field interval into nine subfields (SF0 to SF8) as illustrated in FIG. 17. Each subfield again comprises an addressing interval (X) and a sustaining discharge interval (hatched). The display operation is performed in the manner described above; in each nine-bit code, a ‘1’ causes light to be emitted during the sustaining discharge interval of the corresponding subfield. The lengths of the sustaining discharge intervals are not all proportional to powers of two, however. For example, the ratios of the lengths may be 1:2:4:8:16:32:48:64:80, in order from SF0 to SF8. These values still sum to two hundred fifty-five (1+2+4+8+16+32+48+64+80=255), so two hundred fifty-six gradations from 0 to 255 are displayable by appropriate combinations of light-emitting subfields and non-light-emitting subfields.
With a code in which the bits are weighted according to powers of two, a given graduation is representable by only one pattern of subfields. For example, the only subfield pattern representing gradation 64 is (b7, b6, b5, b4, b3, b2, b1, b0)=(0, 1, 0, 0, 0, 0, 0, 0), and the only subfield pattern representing gradation 128 is (b7, b6, b5, b4, b3, b2, b1, b0)=(1, 0, 0, 0, 0, 0, 0, 0).
With nine-bit codes and bit weights of 80, 64, 48, 32, 16, 8, 4, 2, 1, however, some gradations are representable by a plurality of subfield patterns. For example, there are two (bb8, bb7, bb6, bb5, bb4, bb3, bb2, bb1, bb0) patterns corresponding to gradation 64, namely (0, 0, 1, 0, 1, 0, 0, 0, 0) and (0, 1, 0, 0, 0, 0, 0, 0, 0), and three patterns corresponding to gradation 128, namely (0, 1, 1, 0, 1, 0, 0, 0, 0), (1, 0, 1, 0, 0, 0, 0, 0, 0), and (1, 0, 0, 1, 1, 0, 0, 0, 0). The code converter 11 may operate according to a rule that always assigns the same nine-bit code and thus the same subfield pattern to each gradation value. A sequential arrangement of the subfield patterns assigned to each gradation level from zero to the maximum gradation (in this case, 255) will be referred below to as a ‘subfield sequence’ or simply as a ‘sequence’.
FIG. 18 illustrates one sequence by showing the values of bits bb3 to bb8, which are weighted in the ratios of 8:16:32:48:64:80. (The values of bits bb0, bb1, and bb2 are the same as the values of bits b0, b1, and b2 in an eight-bit code.) The column widths in FIG. 18 are proportional to the bit weights. This sequence always assigns ‘1’ values, indicated by hatching, to bits having the smallest possible weights. The sequence has the following property: if there is a gradation n (0≦n≦254) in which a bit bbx (e.g., bb7) is ‘1’ and the next higher bit bby (e.g., bb8) is ‘0’, and if bit bby is ‘1’ in the next higher gradation (n+1), then bit bbx is ‘0’ in this next higher gradation (n+1). This property will be referred to below as the ‘rollover rule’. In FIG. 18 the rollover rule is obeyed in all bit positions.
FIG. 19 illustrates a moving picture having a rollover at the most significant bit position in the sequence in FIG. 18. The horizontal axis of FIG. 19 again represents pixel position in one raster line on the screen, and the vertical axis represents time. The rollover occurs when the gradation value changes from 175 to 176. The point at which this change occurs is again moving to the left at a rate of two pixels per frame. The viewer's eye follows the motion, so all light emitted at points on dotted lint R0, for example, impinges on the same point on the viewer's retina, and the same is true of lines R1 to R4. FIG. 20 plots perceived luminance as a function of retinal position. The dip at point R3, corresponding to the rollover from subfield SF7 to subfield SF8, is mitigated by the light that continues to be emitted in subfields SF5 and SF6, making the pseudo-contour less noticeable than in FIG. 15. The reason is that the temporal center of gravity of the light emission does not shift as much as in FIG. 15, and there is less total change between the light-emitting and non-light-emitting states. In FIG. 19, the total length of the sustaining discharge intervals in subfields changing from the on-state to the off-state is only 79 (1+2+4+8+64), and the length of the sustaining discharge interval in the single subfield changing from the off-state to the on-state is only 80; in FIG. 15, the corresponding lengths were 127 and 128.
A similar mitigating effect can be obtained from other sequences in which the subfields are arranged in order of increasing (or decreasing) length and their length ratios include values that are not powers of two, particularly if these sequences obey the rollover rule.
FIG. 21 shows a conventional display apparatus that takes a further step toward pseudo-contour mitigation. This display apparatus 101 employs a pair of code converters 12a, 12b, instead of the single code converter 11 in FIG. 13. Both code converters 12a, 12b receive the digital picture signal output by the analog-to-digital converter 3. A code conversion selector 13 controlled by the controller 20 selects the output of one of the two code converters 12a, 12b, and supplies the selected output to the field memory 14.
Code converter A 12a uses the subfield sequence A shown in FIG. 22A, (the same sequence as in FIG. 19); code converter B 12b uses the subfield sequence B shown in FIG. 22B. Both sequences obey the rollover rule.
During operation, the code conversion selector 13 switches between code converter A 12a and code converter B 12b at intervals corresponding to h pixels in the horizontal direction of the screen (h≧1), and v pixels in the vertical direction of the screen (v≧1). Aside from this switching of code converters, the display apparatus 101 in FIG. 21 operates in the same way as the display apparatus 100 in FIG. 13.
FIG. 23 shows the same moving picture as in FIG. 19, displayed by subfield sequence B. Once again, the horizontal axis represents pixel position in one raster line on the screen, and the vertical axis represents time. FIG. 24 illustrates perceived luminance as a function of position on the viewer's retina, points R0, and R1, and R2 receiving light emitted from points on the corresponding dotted lines in FIG. 23. In subfield sequence B, the change from gradation 175 to gradation 176 does not alter the value of any of the three most significant bits (corresponding to subfields SF6, SF7, and SF8); the highest-order rollover occurs in the fourth-highest bit (subfield SF5). The dip in perceived luminance at point R1 is consequently much smaller than the dip at point R3 in FIG. 20.
FIG. 25 plots the subfield sequence selection on the PDP screen for a case in which the selection is switched between sequences A and B at relatively narrow pixel intervals, such as intervals of one pixel in the horizontal direction and one pixel in the vertical direction. FIG. 26 shows an example of perceived luminance as a function of retinal position for a transition from gradation 175 to gradation 176 under these conditions. Sequences A and B produce pseudo-contours at two separate locations on the retina, but the perceived luminance function of each pseudo-contour is visually averaged with the perceived luminance function of the other sequence, so both pseudo-contours are reduced to relatively small dips in the perceived luminance curve.
Further reduction of pseudo-contours is possible by switching among three or more subfield sequences at predetermined intervals.
Although the conventional measures described above succeed in mitigating pseudo-contours in moving pictures, they do not eliminate pseudo-contours, because they do not eliminate rollover at high-order bit positions, including the most significant bit position, where the rollover has the most pronounced effect. A basic problem with these methods is that the same processing is applied to all picture areas, even though pseudo-contours are perceived only in picture areas satisfying certain conditions.